A physical model involving two different crystalline sites for dissolution has been proposed to explain the dissolution behavior of hydroxyapatite and tooth enamel. In this model the properties of individual crystals are considered, along with the appropriate mathematics to describe the diffusional processes occurring when these crystals are assembled into a porous matrix. In this study we will seek to advance our understanding of the properties of the individual crystals and the manner in which these properties affect the observed behavior of the matrix of crystals. Specifically, we will study the kinetics of demineralization and remineralization of hydroxyapatite, along with the accompanying morphological changes at the single crystal level. Experiments will be conducted with synthetic hydroxyapatite crystals, both as slurries and as compressed pellets, in solutions containing fluoride, as well as fluoride-free solutions. Using our two-site model as a guide in the selection of experimental conditions will enable the model to be critically assessed. In the latter phase of this project, the two-side model will be expanded to account for remineralization, as well as demineralization. At this time laboratory experiments with model systems will be conducted in parallel with computer simulation studies to test model hypotheses regarding possible mechanisms for the maturation of enamel and for the development of gradients of fluoride and other ions in maturing enamel.